Check if the coolant is oily?

If not oily I think still not so serious.

Mostly like others said your engine head gasket most likely need replacement, more worrying is if the metal corroded caused leak, that one u need cari potong engine top sudah.

Damage how much? U need open up only know, if lucky few hundreds, if unlucky few thousands.

And check also if the radiator is bubling like they suggest, if bubling really means leaking already, worst thing you don't want it to happen is the water leak in to your combustion chamber or mixed with engine fluid, that's why I asked about if Radiator fluid is oily.